1. Field of the Invention
This invention relates to a sheet feeding apparatus for a recording apparatus which is capable of separating a plurality of cut sheets one by one and conveying them to a recording section.
2. Related Background Art
Generally, some recording apparatuses such as printers have an automatic sheet feeding apparatus in which a recording sheet is set in a sheet insertion port and after a predetermined switch is closed, the sheet is automatically fed to a recording start position.
An example of such apparatus is shown in FIG. 6 of the accompanying drawings. In FIG. 6, the reference numeral 1 designates a carriage on which a ribbon cassette 2 and a thermal head 3 as a recording head are carried. The carriage 1 is guided by a guide shaft 4 and a guide rail 5 and is reciprocally moved in a direction perpendicular to the plane of the sheet. A planar platen 6 formed lengthwisely in a direction perpendicular to the plane of the sheet and having a flat portion 6A which bears against the thermal head 3 is fixed to the opposed surface of the thermal head 3.
The reference numeral 7 denotes a sheet feeding roller rotatably supported on a shaft 8 and rotatively driven by a motor or the like, not shown. A pair of pinch rollers 9-1 and 9-2 are resiliently urged against the lower surface of the sheet feeding roller 7. The reference numeral 10 designates a sheet feeding guide formed integrally with the housing 11 of the printer. The sheet feeding guide 10 slowly comes close to the sheet feeding roller 7 from the left as viewed in FIG. 6, and rotatably supports the pinch rollers 9-1 and 9-2 at the lower end thereof.
In the above-described recording apparatus, when a recording sheet 12 is to be set at the recording start position, the sheet 12 is set onto the sheet feeding guide 10 through a sheet insertion port 13 and the leading end edge thereof is pushed into the nip point 14 between the sheet feeding roller 7 and the pinch roller 9-1. By depressing a switch or the like, not shown, the sheet feeding roller 7 is rotated by a predetermined amount and the leading end edge of the recording sheet is inserted between the thermal head 3 and the platen 6. Thereafter, the thermal head 3 is lowered, and then recording is effected.
In the above-described recording apparatus, recording sheets can be manually set one by one. However, separation and feeding in which a plurality of recording sheets are separated one by one and continuously fed cannot be accomplished, and to accomplish such separation and feeding, it has been necessary to separately provide an exclusive sheet feeding apparatus such as a cut sheet feeder.
There is a recording apparatus unitarily incorporating a cut sheet feeder therein. However, such recording apparatus has suffered from disadvantages of bulkiness and complicated structure.
Also, FIG. 7 of the accompanying drawings is a schematic view showing a force acting on cut sheets in a conventional sheet feeding apparatus.
In FIG. 7, piled cut sheets 71 are urged against a sheet feeding roller 72 with pressure force P1 through a spring-biased pressure plate or the like, and the leading end edges of the cut sheets 71 are held down by a separating projection 73 projectedly provided on the axially divided portion of the sheet feeding roller 72, as shown.
The sheet feeding possibility condition and the dual feed preventing condition when in this state, the sheet feeding roller 72 is rotatively driven in the direction of arrow B can be represented by the following equations (1) and (2): EQU F6=.mu.1.multidot.P1-.mu.2.multidot.P1&gt;R (1) EQU F7=.mu.2.multidot.P1-.mu.3 p1&lt;R (2)
where respective symbols represent the following amounts:
F6: conveying force for the first sheet PA1 F7: conveying force for the second sheet PA1 P1: sheet pressing force PA1 R: riding-over force of the separating projection 73 PA1 .mu.1: coefficient of friction between the sheet feeding roller 72 and the first sheet PA1 .mu.2: coefficient of friction between the first sheet and the second sheet PA1 .mu.3: coefficient of friction between the second sheet and the third sheet
Generally, .mu.2 and .mu.3 are the same, but in some cases, they differ from each other due to the irregularity of the sheets and the state of contact thereof.
Now, the sheets (including various recording media such as postcards, cards and plain paper) have their rigidity greatly varied by the environment (temperature, humidity or the like) and further have their surface condition and coefficient of friction also varied by the environment.
Also in the sheet feeding roller 72, the coefficient of friction is varied by the environment.
At a low temperature and low humidity (for example, of the order of 5.degree. C. and 10%, respectively), the rigidity of the sheets (such as postcards) becomes higher and the riding-over force R increases and the coefficients of friction .mu.2 and .mu.3 between the sheet (such as the postcard) and the sheet (such as the postcard) and the coefficient of friction .mu.1 between the sheet and the sheet feeding roller decrease.
On the other hand, at a high temperature and high humidity (for example, of the order of 35.degree. C. and 90%, respectively), the rigidity of the sheets (such as postcards) becomes lower and the riding-over force R decreases and the coefficients of friction .mu.2 and .mu.3 between the sheet and the sheet and the coefficient of friction .mu.1 between the sheet and the sheet feeding roller increase.
From what has been described above, at a low temperature and low humidity, .mu.1 and .mu.2 are small and R is great in equation (1) and therefore, the sheet (such as the postcard) does not ride over the separating projection 73, but slip becomes liable to occur between the sheet feeding roller 72 and the sheet 71.
On the other hand, at a high temperature and high humidity, .mu.2 and .mu.3 are great and R is small in equation (2) and therefore, a phenomenon that two sheets (such as two postcards) are fed at a time (dual feeding) is liable to occur.
To solve the above-noted problems, it is known to vary the pressure contact force P1 of the sheet 71 with the sheet feeding roller 72 and the amount of overlap H between the separating projection 73 and the sheet feeding roller 72 which determines the riding-over force R of the separating projection 73, to thereby find a shape and dimensions which satisfy the conditions of the aforementioned equations (1) and (2) in all environments.
However, equation (1) which is the sheet feeding possibility condition and equation (2) which is the dual feed preventing condition are conditions contrary to each other as previously described and therefore, it is very difficult to satisfy the both conditions in all environments, and it has been impossible to obtain a stable sheet feeding performance in all environments by the sheet feeding apparatus using the conventional separating projection.
There is also a recording apparatus provided with a sheet feeding apparatus of the type which automatically feeds cut sheets one by one by a feed roller serving also as a separating roller.
In the recording apparatus of such type, there are generally provided a sheet sensor for detecting the leading end edge of a sheet and thereby determining the recording start position, and a discharge roller for continuously discharging sheets on which recording has been effected.
The sheet sensor is for automatically accomplishing the heading (to the recording position) of a sheet, and is generally designed to detect the passage of the leading end edge of the sheet, and then feed the sheet by a prescribed amount, thereby accomplishing the heading of the sheet.
Also, the discharge roller has the same amount of sheet conveyance as that of the feed roller, and is generally driven simultaneously with the feed roller and also takes partial charge of the sheet feeding function.
Now, in the recording apparatus of the above-described type, the separating roller and the feed roller are the same roller and thus, the first and second sheet are continuously conveyed.
This has led to the problem that no gap is present between the two sheets and the leading end edge of the second sheet cannot be sensed by the sheet sensor and the heading cannot be accomplished.